You can keep a poker face, not throw a tantrum and smash your racquet a La Connors/McEnroe, and still panic and lose control of the match, and IMO that sometimes happens with Maria when she's in deep trouble. She can look really anxious, even if she always manages to keep her composure. It didn't happen at the AO final, because Vika took quickly command and there was little that Maria could do, but it certainly happened at the USO SF, where Maria didn't look as mentally strong as Vika (no need to talk about her matches against Serena). But they are both very strong in that department, for me the only difference is that, under stress, Vika may have a certain edge over Maria.
I feel like that in 2012, Vika had the mental edge over Maria, but that this year the tide is changing.
There is two ways you can look at this discussion - compare their mental toughness in matches against each other, and compare their mental toughness across their entire careers. If you were to do the latter, Vika would come up short when you do consider the tantrums (you can't discount them), Maria's greater longevity, comeback from a serious injury and all around mental toughness. A lot of credit has been given to Sam Sumyk for improving Vika's mental game - and that itself evidences that a lot did need to improve. For much of her career, it considered that that was what was holding her back, and it is only really in the past 18 months/2 years that she has become one of toughest in the WTA.
The former is a much more interesting and close discussion. As I've said, I think Vika won that in 2012, but I think the tide has turned after the RG semi-final - Maria had some jitters closing it out, but got over the hurdle.